1. Field of the Invention
The field of the invention is methods and apparatus for improving the output and thermal efficiency of a combination rotary kiln with suspension cyclone preheater system. More specifically, the method and apparatus of the present invention are directed to preheating of dry raw meal before introducing it to the first stage of a multistage suspension cyclone preheater system.
2. Description of the Prior Art
Rotary kilns equipped with suspension cyclone preheater systems have achieved considerable popularity since their introduction in the early 1950's. The standard four stage cyclone preheater system is by far the most popular preheater system. Only a few single and two stage cyclone preheater systems are still in use since they have lower thermal efficiency and lower kiln productivity.
The outstanding advantages of suspension cyclone preheater systems are their low fuel consumption, the absence of moving parts (except for the preheater fan) and high kiln productive capacity in terms of tons of clinker per cubic meter of kiln volume.
In a typical suspension cyclone preheater system the hot gases leaving the rotary kiln flow from the bottom to the top of the four cyclone preheater stages while the raw meal passes from the top to the bottom in a direction opposite to the gas. Thus, in a suspension cyclone preheater system there is both a counter-current and a co-current movement of the gas and raw materials. Heat transfer efficiency is very high and the difference in temperature of raw material and gas after heat transfer in each of the cyclone preheater stages 1, 2 and 3 is only around 30.degree. C. The time taken for the raw material to pass through the cyclone preheater system is well below 30 seconds, and even in this short time span, up to 40% decarbonation of the raw meal is achieved.
As a further development toward increasing productivity of suspension cyclone preheater systems, secondary furnaces/precalcinators have been introduced between the third and fourth cyclone preheater stages, whereby raw meal feed is substantially decarbonated--up to 90% decarbonation.
As a result, the kiln's productive capacity (tons/m.sup.3 of kiln volume) is considerably increased and it is possible to practically double the throughput for a kiln of given diameter as compared to a suspension cyclone preheater system without a precalcinator.
Present day kilns with precalcinators have capacities of over 7000 tons per day. Apart from completely new installations, precalcinators can also be provided for existing suspension cyclone preheater systems combined with a rotary kiln to obtain a significant increase in kiln capacity. The fuel consumption (net kcals/kg clinker) for syspension cyclone preheater systems combined with a rotary kiln with and without a precalcinator is substantially the same. The temperature of the raw meal leaving a suspension cyclone preheater system (with or without precalcinator) is typically maintained at around 830.degree. C., since at higher temperatures there is an increased tendency for build-up of encrustations in the fourth stage cyclone preheater and its discharge pipe.
Also, to minimize the formation of such encrustations, the alkali, chloride and sulfate contents of raw meal are required to be within permissible limits. In practice where alkali and other constituents are marginally on the higher side (or where low alkali cement is required), a portion of the kiln exit gas stream is continuously brought out before its entry into the cyclone preheater system. Such a bypass system serves to void objectionable build-up of alkali and other constituents caused by the internal and external recycling of gases in the cyclone preheater system. Necessarily, however, fuel consumption is higher where exit gases are partially let off.
Typically the temperature of the exit gases from the first stage cyclone preheater of the cyclone preheater system (with or without precalcinator) is around 330.degree. C. In some cement processing installations, the heat content of the exit gases is utilized for drying raw materials before or during grinding. The handling of large volumes of dust-laden low-temperature exit gases, however, is not very convenient. Further, raw grinding mills do not run on a continuous around-the-clock basis and therefore utilization of waste gas heat is on an intermittent basis.
Although installation of a fifth cyclone preheater stage would permit a lower exit gas temperature and an improved fuel efficiency such additional cyclone preheater stage is not considered advantageous because of additional power requirements of a preheater fan for such a stage.
Also electrostatic precipitators are commonly used for collection of dust prior to venting of the exit gases from the cyclone preheater system. Since the relatively dry exit gases are at a temperature of over 300.degree. C., it is a common practice to provide a conditioning tower to cool and humidify the exit gases before they are passed through the electrostatic precipitator. In this way, dust collection efficiency is considerably improved and it is possible to have a smaller electrostatic precipitator.
Over the years a number of attempts have been proposed for improving the productive capacity of rotary cement kilns in terms of the tons of cement clinker produced and to improve the thermal efficiency of the kiln. Examples of prior art methods and apparatus for making cement are disclosed in the following patents:
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 3,212,764 Mueller et al. 3,235,239 Peterson 3,317,201 Heinz-Gertmuller et al. 3,761,582 Lippmann 3,784,389 Hastrup 3,887,388 Christiansen 3,895,955 Kondo et al. 3,925,091 Yoshida et al 3,986,886 Sylvest 4,022,568 Needom 4,043,746 Ritzmann 4,045,162 Christiansen 4,083,676 Ritzmann et al. ______________________________________
Of particular note is the teaching of the Kondo et al. U.S. Pat. No. 3,895,955 which is concerned with the use of waste gases in stages of a cyclone preheater system for controlling the operating temperatures in the rotary kiln and using a fluidized calcinator having a separate heat source to increase the capacity of the rotary kiln and to enable the kiln to be operated continuously and stably for a long period of time.
Also of note is the Sylvest U.S. Pat. No. 3,986,866 which teaches drying of a raw meal slurry utilizing hot gases from a rotary kiln and a separate heater for providing supplementary hot gases to various stages of a multistage cyclone preheater system.
As will be described in greater detail hereinafter the method and apparatus of the present invention differ from the teachings of the prior art patents by providing a separate combustion chamber for preheating the dry raw meal and a dry raw meal cement processing apparatus prior to its being fed into the first stage cyclone preheater of a multistage cyclone preheater system. Moreover, the method and apparatus of the present invention can be utilized with or without an additional heat source or calcinator which is utilized between stages, e.g., the third and fourth stages, of a multistage cyclone preheater system.